OK, so you are using photoshop to produce your card, well it can be done but it's not easy, as photoshop is for images not producing cards.
Open photoshop, but make sure there's no photo present, and select File and click on new. From the window that opens select Preset-International Paper and for size select A4 if that's what you are using. Click OK and a new blank document should appear in Photoshop.
Now open the image you was to use for the card and select Windows-Arrange-Vertically, which will put your image next to the open Document in Photoshop. As the blank Photoshop document is A4 therefore it's 297mm by 210mm, so you need to resize the photo to fit this size. I'm guessing you want to fold the card in the middle, so to resize the photo to fit, click the photo and Select Image-Image size. When this Window opens make sure the Constrain Proportions box is ticked then go to Document-Size-Width and click the dropdown box at the side and select mm for millimeters and change the Width number to 210 or less if required. Then select the Bicubic Sharper from the dropdown box then click OK.
Now you have resized it to fit the Document, select the Move Tool, at the top of your tool palette, right click on the image and hold and drag the photo to the blank photoshop document and position as required. If you are printing as a half card, by folding the document in half, then you need to change rotate the image 180 degrees and reposition at the top. This is because when you print it and fold in half it will appear the right way up.
Hope this helps and you can adjust what I have said as required.
When you select to print in Photoshop a Window will open showing your image on a A4 document. You can tick the Scaled Print Size-Scale to fit media box but it should make any difference as the instructions above have already done this.